This application is based on application Nos. 00-12504 and 014898 filed in the Korean Industrial Property Office on Mar. 13, 2000 and Feb. 1, 2001, the content of which is incorporated herein by reference.
(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery exhibiting good electrochemical properties and a method of preparing the same.
(b) Description of the Related Art
Rechargeable lithium batteries use a material from or into which lithium ions are intercalated or deintercalated as positive and negative active materials. Rechargeable lithium batteries produce electric energy by an oxidation and reduction reaction during the intercalation and deintercalation of lithium ions.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are generally used. Typical examples include LiCoO2, LiMn2O4, LiMnO2, LiNiO2, or LiNi1xe2x88x92xCoxO2(0 less than X less than 1). LiCoO2 provides good electrical conductivity, a high cell voltage of about 3.7V, good cycle life and safety characteristics, and high discharge capacity of 160mAh/g, and thus it is widely used. However, it is very expensive and the cost portion of LiCoO2 reaches to 30% of the total manufacturing cost of the battery. Therefore, it is desirable to develop a low cost positive active material to replace LiCoO2.
Manganese-based materials such as LiMn2O4 or LiMnO2 are easy to prepare, cost less than LiCoO2, are environmentally friendly, and have higher cell voltage (3.9V) than that of LiCoO2. However, the manganese-based materials have a low capacity of about 120 mAh/g, which is smaller than that of LiCoO2 by 20%. Thus, with manganese-based materials it is difficult to fabricate high capacity or thin batteries. LiNiO2 is also lower cost than LiCoO2 and has a high charge capacity, but is difficult to produce. LiNi1xe2x88x92xCoxO2 (0 less than X less than 1) also has a larger capacity (200 mAh/g) than LiCoO2, but a lower discharge potential, inferior cycle characteristics to LiCoO2, and poor safety characteristics.
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery exhibiting good electrochemical characteristics, which is also inexpensive.
It is another object to provide a positive active material for a rechargeable lithium battery exhibiting good thermal stability.
It is still another object to provide a method of preparing the positive active material.
These and other objects may be achieved by a positive active material for a rechargeable lithium battery including at least one compound represented by formulas 1 to 4, and a metal oxide or composite metal oxide layer formed on the compound.                               Li          x                ⁢                  Ni                      1            -            y                          ⁢                  Mn          y                ⁢                  F          2                                    (        1        )                                          Li          x                ⁢                  Ni                      1            -            y                          ⁢                  Mn          y                ⁢                  S          2                                    (        2        )                                          Li          x                ⁢                  Ni                      1            -            y            -            z                          ⁢                  Mn          y                ⁢                  M          z                ⁢                  O                      2            -            a                          ⁢                  F          a                                    (        3        )                                          Li          x                ⁢                  Ni                      1            -            y            -            z                          ⁢                  Mn          y                ⁢                  M          z                ⁢                  O                      2            -            a                          ⁢                  S          a                                    (        4        )            
(where M is selected from the group consisting of Co, Mg, Fe, Sr, Ti, BP Si, Ga, Al, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr, and 0.95xe2x89xa6xxe2x89xa61.1, 0 less than y xe2x89xa60.99, 0xe2x89xa6zxe2x89xa60.5, and 0xe2x89xa6axe2x89xa60.5)
In order to achieve the objects, the present invention provides a method of preparing the positive active material for a rechargeable lithium battery. In the method, at least one compound represented by formulas 1 to 4 is prepared and the compound is coated with a metal alkoxide solution, an organic solution of metal salt or an aqueous solution of metal salt. The coated compound is then heat-treated. The compound represented by formulas 1 to 4 is prepared by co-precipitating a nickel salt and a manganese salt to prepare a nickel manganese salt, mixing the nickel manganese salt with a lithium salt, and then heat-treating the mixture. In the co-precipitation step, a fluorine or sulfur salt may be further used. Alternatively, a salt of metal is further used in the mixing step.